Dynamic knee balancer with pressure sensing

ABSTRACT

A device for performing a surgical procedure on a knee includes an adjustable femoral portion, a tibial portion and at least one sensor coupled with the femoral and/or tibial portions to sense pressure exerted by the femoral and tibial portions against one another. The femoral portion is adapted for removably coupling with a distal end of a femur to adjust tension in soft tissue adjacent the knee and has at least one positioning feature adapted to move relative to the distal end of the femur as the femoral portion is adjusted, thus helping position a femoral prosthetic on the distal end of the femur. The sensor(s) may be adapted to sense pressure at medial and lateral sides of the knee, and relative pressures may be displayed as data on a visual display. Adjustments to the femoral member may be made to balance pressure at flexion and extension of the knee.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/773,608, filed Feb. 6, 2004, the full disclosureof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to medical/surgical devices,systems and methods. More specifically, the invention relates todevices, systems and methods for enhancing a knee surgery procedure.

Total knee replacement surgery, also referred to as total kneearthroplasty (“TKA”), is becoming an increasingly important treatmentfor chronic knee pain and joint dysfunction. A recent panel of theNational Institutes of Health at a Consensus Development Conferencerecognized that approximately 300,000 TKA surgeries are performedannually in the U.S. for end-stage knee arthritis. The NIH panel agreedthat although advances have been made in TKA surgical devices andtechniques, improved outcomes through further innovations should stillbe diligently pursued. The panel concluded that techniques for placingartificial knee prostheses, in particular, should be improved to providebetter outcomes and reduce wear of the prostheses, to thus reduce theneed for repeat TKA surgeries. If advances in TKA continue to be made,the procedure may become more readily available to younger patients,obese patients, and the like, who may need TKA but who do not fallwithin in the “ideal” age range traditionally defined as between 60 and75 years old. Improved techniques and devices would also mean enhancedoutcomes for all TKA patients, with better functioning of the knee jointand longer useful life of the prosthetic knee.

The knee is generally defined as the point of articulation of the femurwith the tibia. Structures that make up the knee include the distalfemur, the proximal tibia, the patella, and the soft tissues within andsurrounding the knee joint. Four ligaments are especially important inthe functioning of the knee—the anterior cruciate ligament, theposterior cruciate ligament, the medial collateral ligament, and thelateral collateral ligament. In an arthritic knee, protective cartilageat the point of articulation of the femur with the tibia has been wornaway to allow the femur to directly contact the tibia. This bone-on-bonecontact causes significant pain and discomfort. The primary goals of aTKA procedure are to replace the distal end of the femur, the proximalend of the tibia, and often the inner surface of the patella withprosthetic parts to avoid bone-on-bone contact and provide smooth,well-aligned surfaces for joint movement, while also creating a stableknee joint that moves through a wide range of motion.

One of the greatest challenges in TKA surgery is to properly balanceligament tension, especially in the medial and lateral collateralligaments, through a full range of motion of the knee. The collateralligaments, which connect the distal femur and proximal tibia on themedial and lateral aspects of the knee, account for much of thestability and movement of the knee. If one of the collateral ligamentsis too lax or too tight relative to the other collateral ligament, theknee will typically be unstable, range of motion may be limited, thepatella may track improperly, and the femur and/or tibia may wearunevenly, leading to arthritis and pain. Uneven ligament tension afterTKA surgery will typically cause joint instability and poor patellartracking, limited range of motion, and impaired function of the knee, aswell as uneven, increased wear of the prosthetic device, which oftennecessitates repeat surgery. Thus, it is imperative for the short- andlong-term success of a TKA procedure to achieve balanced ligamenttension in the knee through a full range of motion.

Balancing ligament tension during TKA surgery is complicated by the factthat the natural knee does not operate like a hinge moving about asingle axis. The knee exhibits dynamic external rotation of the tibiarelative to the femur as the knee moves from its flexed to its fullyextended position. This automatic rotation of the tibia occurs in theopposite direction when the knee is flexed from its fully extendedposition to produce an internal rotation of the tibia relative to thefemur. Thus, the natural knee exhibits a rotary laxity that allows thetibia to rotate through a limited internal and external arc, during kneeflexion. Additionally, the femur translates anteriorly and posteriorlyas the tibia is being flexed about it, bringing yet another movementvariable into the equation. Thus, the ligaments of the knee, along withthe femur, tibia and patella, create a truly dynamic bio-mechanism,making ligament tension balancing in TKA surgery extremely challenging.Many articles and studies have been devoted to ligament tensionbalancing in TKA, such as the following: Mihalko, W H et al.,“Comparison of Ligament-Balancing Techniques During Total KneeArthroplasty,” Jnl. Bone & Jt. Surg., Vol. 85-A Supplement 4, 2003,132-135; Eckhoff, D G et al., “Three-Dimensional Morphology andKinematics of the Distal Part of the Femur Viewed in Virtual Reality,”Jnl. Bone & Jt. Surg., Vol. 85-A Supplement 4, 2003, 97-104; and Ries, MD, et al., “Soft-Tissue Balance in Revision Total Knee Arthroplasty,”Jnl. Bone & Jt. Surg., Vol. 85-A Supplement 4, 2003, 38-42.

One technique for balancing collateral ligament tension during a TKAprocedure involves cutting fibers of one or both ligaments to decreaseligament tension—a technique referred to as “ligament release.” Althoughligament release is still commonly used, the disadvantage of thistechnique is that it requires actually cutting ligament tissue, thusweakening the ligament(s) and leaving less room for error if futurereleases or TKA procedures are required.

Rather than or in addition to ligament release, the components of atotal knee prosthesis may be selected and positioned to balance ligamenttension. Since the femoral and tibial components of the prosthesis areattached to cut surfaces of the distal femur and proximal tibiarespectively, placement and orientation of the bone cuts are alsocritically important. Typically, the tibial component of the prosthesisis positioned on a flat, horizontal cut surface of the proximal tibia(at a 90 degree angle relative to the long axis of the tibia), and theposition and orientation of the tibial component typically do not varygreatly from knee to knee. Therefore, most of the variation inpositioning of the total knee prosthesis typically occurs in positioningthe femoral component and the femoral bone cuts. The surgeon attempts tomake these femoral bone cuts to achieve a position and orientation ofthe femoral prosthetic component so as to optimally balance ligamenttension through a full range of motion of the knee. As with ligamentrelease however, it is often very challenging to position the femoralbone cuts and femoral prosthetic component to provide ideal ligamenttension through the range of motion. This is due primarily to thecomplexity of motion about the knee, as described above, and thedifficulty of placing the femoral component so as to maintain desiredligament tension through the full range of motion. Specifically, therotational, proximal/distal and anterior/posterior orientations andlocations of the femoral component are all critical for duplicating thekinematics of the knee.

In a typical TKA procedure, multiple cuts are made to the distal femurbefore attaching the femoral component of the prosthesis. Mostprocedures, for example, involve making a distal cut across the distalend of the femur, anterior and posterior cuts, and angled anterior andposterior chamfer cuts to help secure the femoral component solidly inplace. In order to effectively and accurately make these resections,orthopedic surgeons typically use a cutting block or cutting guide, usedto guide a surgical saw blade or rotary tool, which is temporarilyattached to the distal end of the femur. Positioning of such a cuttingblock, therefore, is crucial to forming well-positioned bone cuts forattachment of the femoral prosthetic component.

A number of devices and techniques have been described that attempt tofacilitate ligament balancing during a TKA procedure. Some techniques,such as those described in U.S. Pat. No. 5,733,292, involve trialprosthesis components which are used after femoral and tibial bone cutsare made to assess ligament tension. Some devices, such as thosedescribed in U.S. Patent Application Publication No. 2003/0187452, areused to measure a gap between the distal femur and proximal tibia inextension and to help a surgeon recreate that same gap when the knee isin flexion. Other “gap checking” devices are described in U.S. Pat. No.6,575,980. Other devices have been developed to help measure an amountof ligament tension or to apply a desired amount of tension to theligaments. U.S. Pat. No. 4,501,266, for example, describes a kneedistraction device for applying a desired amount of tension. Manypaddle-like devices have been suggested for applying or measuringtension across a knee joint, such as the devices described in U.S. Pat.Nos. 5,597,379; 5,540,696; 5,800,438; 5,860,980; 5,911,723; and6,022,377.

One proposed alternative to the cutting block technique for making bonecuts on a distal femur involves the use of robotic surgical systems formaking distal femoral bone cuts. With robotic surgery and surgicalnavigation, a surgical saw blade or bur is still used, but the bone cutsare positioned as a result of fiducial-based or shape-based registrationof the patient's anatomy. In fiducial-based approaches, fiducials, ormarkers are attached to pertinent anatomical structures prior toimaging. During surgery, the markers are exposed, and a sensor systemconveys their location to the computer. A wide variety of sensingsystems available, including optical trackers, electromagnetictransceivers, articulated probe arms, and ultrasonic and laser rangefinders. In shape-based approaches, the shapes of anatomical structuresare fitted to preoperative image data. The patient measurements can beobtained from a variety of sensing techniques, including tracing curves,scanning distances, or processing images, via one or some of theaforementioned sensing systems. One description of the use of roboticsurgery systems in knee surgery procedures is found in Howe, R D, andMatsuoka, Y, “Robotics for Surgery,” Annu. Rev. Biomed. Eng. 1999,01:211-240.

Although some of the devices and techniques described above have helpedenhance and facilitate TKA procedures, currently available devices andtechniques still have a number of shortcomings. Most importantly,currently available devices do not allow a physician to adjust ligamenttension in a knee and also receive positional information based on thatadjustment that can be used to facilitate completion of the TKA surgery.For example, many currently available devices are applied only inextension or only in flexion of the knee, or must be removed andreplaced when the knee is moved from extension to flexion. Thus, it isdifficult or impossible to assess ligament tension through the fullrange of motion using many currently available devices. Some devicesrely on measuring a gap or amount of tension in extension and thenrecreating the gap or tension in flexion. Again, this does not alwaysresult in collateral ligament balance throughout the range of motion.Still other devices are very cumbersome and/or complex. Many includelarge parts which fit external to the knee joint and necessitate thepatella being moved to the side during measurement or other phases ofthe TKA procedure. Furthermore, current devices typically do not resideprimarily within the joint space during a surgical procedure to allowfor the natural movements, rotations and translations of the tibia andfemur as the knee is flexed through a range of motion. In sometechniques, bone cuts are made before ligament balancing is achieved,thus often requiring re-cutting of those same bone cuts. More bone cutsmean more trauma to the patient, a longer recovery period, and less boneto work with if a second TKA is required later in life.

Although robotic surgery may provide a level of improvement over moretraditional techniques, it is typically difficult or impossible usingcurrent robotic techniques to dynamically mark or register and sense theproper dynamic position to make well-positioned, subsequent bone cutsfor attachment of the femoral prosthetic component. Thus, even withrobotic systems, it is still challenging to achieve a desired ligamentbalance to enhance knee stability, range of motion and patellartracking. These and other shortcomings of currently available devicesand methods continue to make ligament balancing, and specificallycollateral ligament balancing, one of the most challenging aspects ofTKA surgery.

Therefore, a need exists for improved devices, systems and methods forenhancing TKA surgery and specifically for dynamically balancingligaments during TKA to improve range of motion, stability, and patellartracking of the prosthetic knee joint. Ideally, such devices would helpa surgeon balance ligaments dynamically, through a full range of motionof the knee, allowing for the natural rotation of the tibia and thenatural translation of the femur while the tibia is being flexed aboutit. Also ideally, such devices and methods would allow a surgeon toachieve a desired ligament tension balance before committing to andmaking final bone cuts to the femur. Such devices would ideally besimple to use in conjunction with cutting guides, saw blades or burs,and robotic and navigational systems, preferably allowing the patella toremain in place during assessment of ligament tension. At least some ofthese objectives will be met by the present invention.

BRIEF SUMMARY OF THE INVENTION

The present invention provides devices, systems and methods forenhancing knee surgery procedures, and more specifically total kneereplacement procedures (total knee arthroplasty, “TKA”). Variousembodiments generally include a femoral portion, a tibial portion andone or more sensors for sensing pressure exerted by the femoral andtibial portions against one another. The femoral portion is adjustable,to allow a user to adjust tension in soft tissues adjacent the knee,thus causing changes in the sensed pressure. The adjustable femoralportion also helps a user determine positioning of at least part of aknee prosthesis.

Typically, the adjustable femoral portion is separately adjustable onopposite lateral sides to adjust tension in soft tissues on either orboth sides of the knee, such as the lateral and/or medial collateralligaments. When the adjustable femoral portion is adjusted to adjustligament tension, one or more positioning features of the femoralportion provide positioning information to help position and/or orient acutting guide, surgical saw blade, bur, mill, surgical navigationsystem, robotic surgical system or the like. This positioninginformation is then typically used to make subsequent bone cuts to thedistal femur, or to otherwise mill or shape the distal femur, so thatwhen a femoral prosthetic component is applied, the knee has a desiredstability, range of motion and/or patellar tracking. The sensor (ormultiple sensors) of the device helps a user balance pressure betweenthe femoral and tibial members while the knee is in flexion withpressure while the knee is in extension. If pressure(s) in extension tonot match those in flexion, the user may choose to adjust the femoralmember, thus adjusting soft tissue tension and also pressure between thefemoral and tibial portions of the device. Thus, devices and methods ofthe invention help a user dynamically balance ligament tension in a kneeduring TKA surgery, without requiring ligament releases, to provide fora dynamically balanced knee after the surgery is complete.

For purposes of the present description, the terms “ligaments of theknee,” “ligaments in the knee,” “ligaments adjacent the knee,” and thelike are all synonymous and all refer generally to any ligaments withinthe knee joint space, around the knee, adjacent the knee, or near theknee. These terms typically refer to the ligaments that assist in thefunctioning of the knee, and often the ligaments referred to are themedial collateral ligament, the lateral collateral ligament, theanterior cruciate ligament and the posterior cruciate ligament. “Softtissues adjacent the knee” or “soft tissues of the knee” include theligaments described above as well as muscles, tendons and other softtissues adjacent and/or in the knee. Although the following descriptionfocuses on the use of various devices and methods in TKA surgicalprocedures, some embodiments may suitably be used to facilitate otherknee surgery procedures and/or other orthopedic joint surgeryprocedures.

That being said, in one aspect of the present invention, a device forperforming a surgical procedure on a knee includes an adjustable femoralportion, a tibial portion and at least one sensor coupled with at leastone of the femoral and tibial portions to sense pressure exerted by thefemoral and tibial portions against one another. The femoral portion isadapted for removably coupling with a distal end of a femur to adjusttension in soft tissue adjacent the knee and has at least onepositioning feature adapted to move relative to the distal end of thefemur as the femoral portion is adjusted, thus helping position afemoral prosthetic on the distal end of the femur. The tibial portion isadapted for removably coupling with a proximal end of a tibia andmovably coupling with the femoral portion to allow the knee to be movedthrough a range of motion without removing the femoral and tibialportions from the knee. Typically, the pressure exerted by the femoraland tibial portions against one another is caused, at least in part, bysoft tissues adjacent the knee. In some embodiments, the pressure may beincreased or decreased on one or both lateral sides of the knee byadjusting the femoral portion of the device.

The sensor(s) may be coupled with the tibial portion, the femoralportion or both, in various embodiments. In a preferred embodiment,sensors are coupled with only one of the two portions, to simplify thedevice design and function, but other embodiments may include sensors onboth portions. The sensors may have any suitable shape, size andconfiguration. One embodiment includes a single sensor comprising alayer of pressure sensing material disposed along a surface of thefemoral portion or the tibial portion to contact the distal femur orproximal tibia. In other embodiments, multiple sensors are used.Optionally, the device may further include a sensor housing platecoupled with the femoral or tibial portion and adapted to house one ormore sensors. Such a sensor housing plate may be either permanently orremovably coupled with the femoral portion or the tibial portion.Optionally, a connector plate may also be included for coupling thesensor housing plate with the femoral portion or the tibial portion. Insome embodiments, the sensor housing plate is adapted to contact thefemur or tibia, thus residing between the bone and the rest of thefemoral or tibial portion. In other embodiments, the plate may bedisposed out of contact with the bone.

Any suitable pressure- or force-sensing material or combination ofmaterials may be used to form the sensor(s), and the sensors themselvesmay have any of a number of configurations, shapes and size. Someexamples of sensors that may be used include, but are not limited topiezoelectric sensors, force sensing resistors, strain gauges, loadcells, other pressure sensors and other force sensors. In someembodiments, the device further includes a processor coupled with thesensor(s) for processing sensed pressure data into usable data forproviding to a user. Typically, though not necessarily, such anembodiment will also include a visual display coupled with the processorfor displaying the usable data. In one embodiment, for example, thevisual display comprises a digital display for providing at least one ofalpha and numerical data to the user, with the device further comprisingan analog to digital converter. In some embodiments, the visual displayseparately displays usable data representing pressure on a lateral sideand a medial side of the knee.

Typically, the knee surgery device will also include one or moreconnectors for connecting the sensor(s) with the visual display and/orthe processor. Connectors such as electrical cable, wireless infrared,electromagnetic and optical connectors may be used, as well as any othersuitable connectors. In some embodiments, the visual display is directlyattached to the femoral portion or the tibial portion, thusnecessitating relatively short connector(s). In alternative embodiments,the visual display is removably couplable with a leg of a patient belowthe knee or above the knee, thus employing longer connector(s). In someembodiments, the device further includes one or more pressure selectionmembers coupled with the sensor(s) and the femoral portion. The pressureselection member(s) are adapted to allow a user to select a desiredamount of pressure exerted between the femoral and tibial portions andto automatically adjust the femoral portion to achieve the selectedamount of pressure. In some embodiments, the pressure selectionmember(s) are adapted to allow the user to separately select desiredpressures exerted between the femoral and tibial portions at medial andlateral sides of the knee.

In one embodiment of the device, the femoral portion includes astationary femoral member for removably attaching in a fixed position tothe distal end of the femur and a mobile femoral member movably coupledwith the stationary femoral member to adjust the tension in the softtissue adjacent the knee. In some embodiments, the mobile femoral memberis separately adjustable on laterally opposite sides of the femoralportion. In such embodiments, adjusting on one lateral side relative tothe opposite side may cause the mobile femoral member to rotate relativeto the distal femur. The positioning feature(s) of the adjustablefemoral member may include, but are not limited to, apertures, drill bitguides, surface markers, surface features, measurement devices, embeddedmarkers, fiducials, transponders, transceivers and/or sensors. Invarious embodiments, the positioning feature(s) may serve a number ofdifferent functions for a user, such as facilitating placing a cuttingguide on the distal femur for making bone cuts, making one or more bonecuts on the distal femur, or positioning a prosthetic femoral componenton the distal femur.

In some embodiments, the tibial portion comprises at least one shim,paddle, plate, bar, platform or rod. In one embodiment, the tibialportion comprises a plurality of tibial shims having differentthicknesses or heights, and any one of the plurality of shims may beselected for engaging with the proximal end of the tibia to provide adesired amount of tension in soft tissue adjacent the knee. In someembodiments, the femoral and tibial portions are movably coupled viaforce provided by the soft tissue adjacent the knee. Also in someembodiments, the femoral and tibial portions may be adapted to resideprimarily within a joint space between the distal end of the femur andthe proximal end of the tibia. In some embodiments, the patella of theknee remains approximately in its anatomical position while the femoraland tibial portions are engaged and the knee is moved through the rangeof motion.

In another aspect of the invention, a device for performing a surgicalprocedure on a knee includes a femoral portion for removably couplingwith a distal end of a femur, a tibial portion for removably couplingwith a proximal end of a tibia and movably coupling with the femoralportion to allow the knee to be moved through a range of motion withoutremoving the femoral and tibial portions from the knee, means foradjusting the position of the femoral portion relative to the tibialportion to adjust tension in soft tissue adjacent the knee, and at leastone sensor coupled with at least one of the femoral and tibial portionsto sense pressure exerted by the femoral and tibial portions against oneanother.

In yet another aspect of the present invention, a system for performinga surgical procedure on a knee includes a knee adjustment deviceincluding an adjustable femoral portion and a tibial portion, and asensor device coupled with the femoral or tibial portion. The adjustmentdevice may include any of the features described above. The sensordevice includes at least one sensor coupled with the femoral or tibialportion to sense pressure exerted by the femoral and tibial portionsagainst one another, a processor coupled with the sensor(s) forprocessing sensed pressure data into usable data for providing to auser, and a visual display coupled with the processor for displaying theusable data. The sensor(s), processor and visual display may include anyfeatures and combinations described above.

In another aspect of the present invention, a method for sensingpressure in a knee during a surgical procedure on the knee involvesengaging femoral and tibial portions of a knee adjustment device withthe knee, sensing pressure exerted by the femoral and tibial portionsagainst one another, using at least one sensor coupled with the femoralor tibial portion, displaying data describing the sensed pressure, andmoving the knee through a range of motion while the knee adjustmentdevice remains engaged with the knee. Optionally, the method may furtherinvolve adjusting the femoral portion of the knee adjustment device toadjust tension in soft tissue adjacent the knee. Adjusting the tensionaffects the pressure exerted by the femoral and tibial portions againstone another. In various embodiments, the adjusting step may be performedbefore the moving step, after the moving step, or both. In someembodiments, the adjusting step is performed one or more times tobalance pressure data displayed while the knee is flexed with pressuredata displayed while the knee is extended. In an alternative embodiment,tension/pressure adjustment is achieved by inserting and removingmultiple differently-sized tibial portions. Again, this may be performedbefore moving the knee, after moving the knee, or both. In oneembodiment, various tibial portions are used to balance pressure whenthe knee is flexed with pressure when the knee is extended. In someembodiments, adjustments may be made to both the femoral and tibialmembers.

Typically, though not necessarily, the method also involves processingthe sensed pressure into the data describing the pressure. In someembodiments, for example, processing the data involves converting analogdata to digital data. Pressure sensing may be achieved in a number ofdifferent ways. In one embodiment, for example, sensing the pressureinvolves transmitting a voltage to at least one sensor, measuring thevoltage after it has passed through the sensor(s), determining apercentage of the voltage passed through the sensor(s) relative to thevoltage transmitted to the sensors, and deriving the pressure from thepercentage. In other embodiments, sensing may involve using one or moresensors such as but not limited to piezoelectric sensors, force sensingresistors, strain gauges, load cells, other pressure sensors or otherforce sensors.

In one embodiment, displaying the data involves displaying at least afirst number representing the pressure in a medial portion of theadjustment device and displaying at least a second number representingpressure in a lateral portion of the adjustment device. Optionally, themethod may further involve receiving an input from a user of a desiredamount of pressure to be exerted by the femoral and tibial membersagainst one another and automatically adjusting the femoral portion ofthe adjustment device to achieve the desired amount of pressure. In someembodiments, receiving the input includes receiving a first amount ofdesired pressure for a medial side of the knee and a second amount ofdesired pressure for a lateral side of the knee. Some embodimentsoptionally include automatically adjusting the adjustment device tobalance the pressure exerted by the femoral and tibial members againstone another while the knee is flexed and extended.

Further details of these and other embodiments are described more fullybelow, with reference to the attached drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a frontal view of a knee in extension, with a knee balancingdevice according to one embodiment of the invention in place within theknee joint;

FIG. 1B is a side view of the knee in extension and knee balancingdevice shown in FIG. 1A;

FIG. 1C is a side view of the knee and knee balancing device shown inFIGS. 1A and 1B, with the knee in a position of flexion;

FIG. 1D is a side view of the knee and knee balancing device shown inFIGS. 1A-1C, with the knee balancing device adjusted to achieve adesired ligament tension balance according to one embodiment of theinvention;

FIG. 1E is a frontal view of the knee and knee balancing device shown inFIGS. 1A-1D, with the knee balancing device adjusted to achieve adesired ligament tension balance according to one embodiment of theinvention;

FIG. 2A is a frontal view of a knee balancing device according to oneembodiment of the present invention;

FIG. 2B is a rear view of the knee balancing device shown in FIG. 2A;

FIG. 2C is a side view of the knee balancing device shown in FIGS. 2Aand 2B;

FIG. 3A is a front-perspective view of a knee balancing device accordingto one embodiment of the present invention;

FIG. 3B is a rear-perspective view of the knee balancing device shown inFIG. 2A;

FIG. 3C is a front-perspective view a knee balancing device according toanother embodiment of the present invention

FIG. 4A is a front-perspective, exploded view of a knee balancing deviceaccording to one embodiment of the present invention; and

FIG. 4B is a rear-perspective, exploded view of the knee balancingdevice shown in FIG. 4A.

FIG. 5 is a front-perspective view of a knee balancing device withsensing capability, including a visual display and shown with anadjustment member for adjusting the femoral portion of the balancingdevice according to one embodiment of the present invention.

FIG. 6 is a front-perspective view of the tibial portion and visualdisplay of the device of FIG. 5, with the tibial portion shown inexploded view.

FIG. 7 is a superior, angled perspective view of the tibial portion andvisual display of FIGS. 5 and 6.

FIG. 8 is a perspective view of a tibial portion of a knee balancingdevice with sensing capability coupled with a visual display accordingto an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, the present invention provides devices, systems andmethods primarily intended for enhancing total knee arthroplasty (TKA)surgical procedures. Although these devices, systems and methods areused primarily in TKA, however, some embodiments may be used to enhanceother knee surgery procedures or surgical procedures on other joints,such as an elbow joint.

That being said, devices, systems and methods of the invention generallyhelp a surgeon to balance ligament tension in a knee during a TKAprocedure and thereby help the surgeon perform the TKA so as to achievea desired ligament balance when the surgery is complete. Devices,systems and methods of the invention generally facilitate dynamicbalancing of ligaments of the knee, such that these ligaments remainbalanced through a range of motion about the knee. Oftentimes, suchdynamic balancing helps create a prosthetic knee that has a desirablelevel of stability, patellar tracking and range of motion.

With reference now to FIG. 1A, a frontal view of a right knee K is shownin extension, with a knee balancing system 10 in place within the kneejoint space. The anatomical components of the knee K that are pertinentto this description include a distal femur F, a proximal tibia T, amedial collateral ligament MCL, and a lateral collateral ligament LCL.(Also labeled is the proximal fibula Fi, to which the LCL attaches.) Theknee K is shown without a patella, medial collateral ligament or lateralcollateral ligament, for clarity, but many embodiments may be used whilethe patella is in its anatomical position on the anterior aspect of theknee K. In FIG. 1A, a portion of the distal end of the distal femur Fand a portion of the proximal end of the proximal tibia T have been cutor shaved off, to create level surfaces on which to place femoral member12 and a tibial member 14, respectively, of dynamic knee balancingsystem 10. In various embodiments, a knee balancing device may beprovided as only a femoral member, for example to be used withoff-the-shelf tibial trial inserts. In other embodiments, knee balancingsystem 10, comprising femoral member 12 and tibial member 14 may beprovided.

In the embodiment shown, femoral member 12 is adjustable to adjusttension in the MCL, the LCL, or both. Adjustability may be achieved byany suitable means, some of which are described in more detail above andbelow. In one embodiment, for example, one or more adjustment members16, which may comprise screws, pins, levers, spring-loaded mechanisms,shape memory materials or the like, are coupled with femoral member 12to provide adjustability. In some embodiments, adjustment members 16 maybe used for separately adjusting femoral member 12 on either side toseparately adjust tension in the MCL or the LCL.

In general, femoral member 12, tibial member 14 and any of theircomponent parts may be manufactured from any suitable material now knownor hereafter discovered. For example, femoral member 12 and/or tibialmember 14 in some embodiments may be manufactured from one or moreplastics, composites and/or metals, such as aluminum, stainless steel,composite, cobalt-chrome, titanium, or the like. These or any othersuitable material(s) and combinations of materials may be used invarious embodiments.

As shown in FIG. 1A and subsequent figures, knee balancing system 10 istypically disposed primarily within the joint space of knee K during aTKA surgery, thus providing for more convenient manipulation of theknee, anatomical positioning of the patella during surgery and the like.In alternative embodiments, however, a knee balancing device or systemcould be engaged with the knee at a location external to the knee joint.For example, in one embodiment the device may comprise an externallyapplied frame that performs the same functions as the devices describedherein. In such embodiments, some or all of the knee balancing devicemay be located external to the knee joint, thus not fitting within theknee joint space during the surgical procedure.

Referring now to FIG. 1B, the knee K is shown from a side view. In thisand subsequent figures, the collateral ligaments MCL and LCL, otherligaments such as the posterior cruciate ligament PCL, and the fibula Fiare removed for clarity. As is visible in this view, femoral member 12suitably comprises a stationary femoral member 18 and an adjustablefemoral member 17. Stationary femoral member 18 is typically removablyattached to a surface of the distal femur F, often a cut surface at thedistal end of the distal femur F, and adjustable femoral member 17 iscoupled with stationary femoral member 18. Stationary femoral member 16includes at least one stationary posterior condylar member 18′ extendingposteriorly to contact at least one of the medial and lateral posteriorcondyles PC of the distal femur F. Typically, stationary femoral member18 includes two stationary posterior condylar members 18′, one for eachposterior condyle PC. Similarly, adjustable femoral member 17 suitablyincludes one or more (preferably two) adjustable posterior condylarmembers 17′ extending posteriorly emulate the two posterior condyles PC.As is described more fully below, posterior condylar members 17′, 18′allow femoral member 12 to be adjusted to balance ligament tension inthe knee K and also allow knee balancing system 10 to remain in placewithin the joint space while the knee K is moved through a range ofmotion. In various embodiments, stationary femoral member 18 andstationary posterior condylar members 18′ may be either multiple, coupleparts or may be one piece or extrusion. Similarly, adjustable femoralmember 17 and adjustable posterior condylar members 17′ are all onepiece or extrusion in some embodiments, but may alternative comprisemultiple coupled parts.

Typically, adjustable femoral member 17 is movably engageable withtibial member 14 to allow knee balancing system 10 to remain in placewithin the knee joint space while the knee K is moved through a range ofmotion. In some embodiments, such as the one shown in FIG. 1 andsubsequent figures, adjustable femoral member 17 and tibial member 14are movably engaged with one another via force applied by the ligamentsof the knee K, especially the MCL and LCL. In other words, femoralmember 12 and tibial member 14 are two separate components which arebrought together into a movable/slidable coupling by ligament force.Such coupling of adjustable femoral member 17 and tibial member 14 vialigament force provides for dynamic balancing of the knee through a fullrange of motion. In various alternative embodiments, ligament force maynot be used for coupling femoral member 12 with tibial member 14, andinstead a passive mechanical coupling may be used.

With reference now to FIG. 1C, knee balancing system 10 is shown withthe knee K in flexion. It can be seen here that stationary posteriorcondylar member 18′ and adjustable posterior condylar member 17′ areslidably engageable with complementary grooves 20 on tibial member 14.Thus, knee balancing system 10 is movable/slidable through approximatelya full range of motion of the knee K, from full extension to fullflexion and vice versa.

Referring to FIG. 1D, knee balancing system 10 is shown after anadjustment has been made to adjustable femoral member 17. In oneembodiment, adjustable femoral member 17 is separately adjustable oneither side to separately adjust tension in the MCL and/or the LCL. Suchadjustment(s) may be achieved by any suitable means, such as manualadjustment via a screw or other adjustment member, self-adjustment via aspring-loaded mechanism, or the like. In the embodiment shown,adjustment member 16 is adjusted to move adjustable femoral member 17relative to stationary femoral member 18. As adjustment member 16 isadjusted, adjustable femoral member 17 rotates relative to stationaryfemoral member 18, thus causing adjustable posterior condylar member 17′to move away from stationary posterior condylar member 18′. Thismovement creates a larger joint space on the side of adjustment, thustightening the collateral ligament on that side. Meanwhile, the distalfemoral portion of adjustable femoral member 17 has rotated relative tothe distal femoral portion of stationary femoral member 18,approximately about the long axis of the femur F. If adjustment members16 on both sides of adjustable femoral member 17 are adjusted in thesame direction, adjustable femoral member 17 may be caused to moveanteriorly or posteriorly relative to stationary femoral member 18.Thus, adjustable femoral member 17 may be adjusted rotationally as wellas in an anterior/posterior orientation.

With reference now to FIG. 1E, the knee K and knee balancing system 10of FIG. 1D is shown in frontal view. Here it can be seen that adjustmentof adjustment member 16, on the lateral side of the distal femur F, hascaused adjustable posterior condylar member 17′ on the lateral side tomove away from stationary posterior condylar member 18′ on the lateralside, thus increasing the height of the joint space on the lateral sideand rotating adjustable femoral member 17 slightly, relative to thedistal femur. Adjustable femoral member 17 includes at least onepositioning feature for providing positional information forfacilitation the TKA procedure. As described above, the positioningfeature(s) may include any of a number of different features, such asapertures, surface markers, embedded markers, fiducials, transmitters,transponders, transceivers, sensors and/or the like. These positioningfeatures provide positional information that can then be used tofacilitate the TKA procedure. For example, apertures may act as drillbit guides for drilling holes to apply a cutting guide to the femur F tomake subsequent bone cuts. In another embodiment, apertures may containfiducials or markers to provide information to a navigational systemand/or robotic surgical system for positioning subsequent bone cuts orotherwise shaping the distal femur F via milling, burring or the like.Various embodiments have been fully described above, and any suitablepositioning features and positional information may be used in variousembodiments.

In the embodiment shown, adjustable femoral member 17 includes twoapertures 24 as positioning features. Apertures 24 extend throughadjustable femoral member 17 and also through stationary femoral member18 such that apertures 24 may be used to guide a drill bit to form holesin the distal femur F. Of course, as just discussed, apertures 24 canserve any of a number of other functions, such as carrying fiducials,sensors, markers or the like. In some embodiments, correspondingapertures in stationary femoral member 18 are large enough to allow formovement of apertures 24 on adjustable femoral member 17 such thatapertures 24 extend all the way to the cut surface of the distal femurF. When apertures 24 are used to drill holes for a cutting guide, thebalancing system 10 is removed, holes are used to attach a cutting guideto the distal femur F, and the cutting guide used to make subsequentbone cuts on the femur F. Once these bone cuts are made, a femoralprosthetic component is typically placed on the cut distal end of thefemur. These final bone cuts thus determine the position and orientationof the femoral prosthetic component. Alternatively, positioninginformation may be used to orient/position bone cuts by some other means(not using a cutting guide), such by guiding a saw blade, rotary cutter,bur or the like to make the actual bone cuts. In some embodiments,position information may be used to guide a robotic surgical system, toenhance the procedure via a navigational system, or the like.

Also shown in FIG. 1E are two stationary femoral member attachmentscrews 22. These screws are used to removably attach stationary femoralmember 18 to the distal femur F. Any other suitable attachment device(s)may be used instead of or in addition to attachment screws 22 to attachstationary femoral member 18 to the distal femur F For example,adhesives, pins and/or the like may be used in some embodiments.

FIGS. 2A-2C are anterior, posterior and side views, respectively, of anembodiment of femoral member 12. These figures show two screw holes 23used for attaching stationary femoral member 18 to a distal femur. Theyalso show drill guide apertures 24 which are formed by bushings 26coupled with adjustable femoral member 17 and stationary femoral member18. Bushings 26 move along slots 27 in stationary femoral member 17 asfemoral member 12 is adjusted.

With reference now to FIGS. 3A and 3B, anterior and posteriorperspective views, respectively, of an embodiment of a knee balancingsystem 100 are shown. Knee balancing system 100 suitably includes afemoral member 140 and a tibial member 120. Femoral member 140 mayfurther include an adjustable femoral member 170 having adjustableposterior condylar members 170′ and a stationary femoral member 180having stationary posterior condylar members 180′. In some embodiments,adjustable femoral member 170 and adjustable posterior condylar member170′ will be one unitary piece or extrusion, while in other embodimentsthey may be two or more coupled pieces. Similarly, stationary femoralmember 180 and stationary posterior condylar member 180′ may comprise aone-piece construction or multiple pieces coupled together. In theembodiment shown, stationary femoral member 180 comprises a distalfemoral plate coupled with two stationary posterior condylar members180′. Any suitable configuration, combination or manufacturing processmay be used in various embodiments.

Femoral member 140 may further include adjustment screw holes 161 foringress/egress of adjustment screws (not shown), attachment screws 220,drill guide apertures 240, bushings 260, slots 270 and/or any otherfeatures described previously above. Tibial member 120 may suitablyinclude two grooves 200 or depressions to provide for slidable couplingwith femoral member 140. Generally, any of the features described abovemay be applied to knee balancing system 100.

Referring now to FIG. 3C, a knee balancing system 300 similar to thatdescribed above is shown in frontal-perspective view. System 300includes a tibial member 320 and a femoral member 340, the femoralmember 340 including an adjustable member 370 coupled with a stationarymember 380. Adjustable member 370 includes two adjustable posteriorcondylar members 370′, and stationary member 380 includes two stationaryposterior condylar members 380′. In FIG. 3C, one adjustment member 360 ahas been adjusted to move adjustable posterior condylar portion 370′away from stationary posterior condylar member 380′ on that side, whichwould increase the height of the joint space on that side if the devicewere in a knee joint, and would also rotate adjustable femoral member370 slightly relative to the distal femur. The pictured embodimentincludes two apertures 345 as positioning features, and disposed withinapertures 345 are two fiducials 390 (or markers, sensors or the like)for providing positional information to a computer navigation system orrobotic surgery system. Such positional information, for example, mayinclude a dynamically balanced orientation of the knee to makesubsequent bone cuts on the femur F.

With reference now to FIGS. 4A and 4B, the embodiment of knee balancingsystem 100 from FIGS. 3A and 3B is shown in exploded view to moreclearly show its component parts. In this embodiment, the componentparts of knee balancing system 100 are the same as those shown anddescribed above in reference to FIGS. 3A and 3B. It can be seen in FIGS.4A and 4B that stationary femoral member 180 may comprise three coupledparts—a stationary femoral member distal plate 180 and two stationaryposterior condylar members 180′. Such parts may be coupled by anysuitable means, such as pressure fitting, sandwiching condylar members180′ between plate 180 and adjustable femoral member 170, screws,adhesives, and/or the like. Alternatively, stationary femoral member 180may comprise one unitary piece or extrusion.

An additional part shown in FIG. 4B is a bias spring 392. Bias spring392 may be incorporated into femoral member 140 to allow for rotation ofadjustable femoral member 170 relative to stationary femoral member 180.Alternative embodiments of knee balancing system 100 may include anyother suitable mechanism for allowing such rotation, anterior-posterioradjustment, and/or any other suitable adjustment(s).

In an exemplary method for enhancing a TKA procedure, a femoral memberis typically removably engaged with a distal femur of a knee. Usually,the distal femur will have been cut to form a surface for engaging thefemoral member, but this is not required in all embodiments. A tibialmember is also engaged with a proximal tibia of the knee, usually a cuthorizontal surface of the tibia. This tibial member may be provided aspart of a dynamic knee balancing system or may be an off-the-shelftibial trial insert, in various embodiments. In different embodiments,the tibial member may be placed before the femoral member or vice versa.In one embodiment, the femoral and tibial members are engaged with thefemur and tibia while the knee is in full or nearly full extension,though in alternative embodiments they may be placed in flexion. Theheight, thickness, or overall shape of the tibial member may often beselected to provide a desired amount and balance of ligament tensionwhile the knee is in extension.

Generally, the knee is then moved from extension to flexion, and thefemoral member is adjusted to adjust tension in the MCL, LCL, posteriorcruciate ligament and/or other ligaments to achieve a desired ligamentbalance in flexion. The knee may then be moved through a range ofmotion, and one or more subsequent adjustments may be made to thefemoral member to adjust and balance ligament tension through the rangeof motion. Most, if not all, such adjustments and movements may, in someembodiments, be possible while the patella of the knee remains inapproximately its normal anatomical position over the knee. This isadvantageous because patellar tracking, an important determinant of kneefunction, may be assessed and adjusted during the TKA procedure.Typically, the goal of the surgeon will be to achieve dynamic balancingof ligament tension through the range of motion of the knee. Once thisbalancing is achieved with the femoral and tibial members in place, thepositioning feature(s) on the adjustable femoral member providepositional information to a surgeon, computer, robotic system and/or thelike, to help facilitate completion of the TKA procedure. Using thispositional information, subsequent cuts (or drilling, burring or othershaping methods) are applied to the femur, with such cuts/shapingdetermining how the femoral prosthetic component of the artificial kneejoint will be positioned and oriented on the distal femur. The femoralprosthetic component is then placed accordingly.

Referring now to FIG. 5, another embodiment of a knee surgery system 400generally includes a tibial portion 402, an adjustable femoral portion404, a visual display 420 and an adjustment tool 410 for adjustingfemoral portion 404. Tibial portion 402, which is engaged with aproximal end of the tibia T, includes a sensor plate 412, an adaptor414, and a tibial insert 416. Sensor plate 412 is coupled with visualdisplay 420 via a cord 418. Visual display 420 includes two LED readouts424 and a strap 422 for removably attaching visual display 420 to apatient's leg L. Femoral portion 402, which is engaged with a distal endof the femur F, includes two adjustment screws 408 and two positioningapertures 406. Aside from the sensing and visual display components andfunction, the general operation of tibial portion 402 and femoralportion 404 have been described in detail above.

With reference to FIG. 6, an exploded view of tibial portion 402 isshown. As illustrated, in some embodiments, sensor plate 412 acts as ahousing for one or more sensors 430. Sensors 430 may be any suitableforce or pressure sensors, such as but not limited to piezoelectricsensors, force sensing resistors, strain gauges, load cells or the like.In some embodiments, two sensors 430 are used, in order to sensepressure on medial and lateral sides of the knee. In other embodiments,only one sensor 430 may be used, more than two sensors 430 may be used,sensors 430 may be coupled with both tibial portion 402 and femoralportion 404 and/or the like. In one alternative embodiment, for example,sensor plate 412 itself is one large pressure sensor 430, rather than ahousing for sensors 430. Any suitable combination, shape, size andconfiguration of pressure and/or force sensors is contemplated.

Adaptor 414 is generally a plate coupled with sensor plate 412 andadapted to couple sensor plate 412 with tibial insert 416. Typically,adaptor plate 414 is removably couplable with tibial insert 416, suchthat multiple, differently-sized inserts 416 may be tried in the kneeduring a surgical procedure, while using the same sensor plate 412 andadaptor 414. In some embodiments, such as the one shown in FIG. 6,adaptor 414 and sensor plate are two pieces attached together. Inalternative embodiments, a one-piece plate may be used to house sensors430 and to couple with tibial inserts 416. In yet another embodiment,all of tibial portion 402 may be one piece. Furthermore, it is notrequired that sensor plate 412 be located in contact with the tibia T.In an alternative embodiment, for example, sensor plate 412 may bedisposed within a tibial insert 416 so as not to contact the tibia T. Inthe embodiment shown, sensors 430 are embedded in sensor plate 412, andadaptor 414 is attached to sensor plate 412 via adhesive, welding or anyother suitable method.

As previously mentioned, sensors 430 may comprise any of a number ofsuitable pressure and/or force sensors. In one embodiment, a knownvoltage is transmitted to sensors 430, the voltage passing out ofsensors 430 is measured, and a percentage of the voltage leaving sensors430 to the known voltage is calculated. From this percentage, pressureis derived. An analog signal representing the pressure is converted to adigital signal with an analog-to-digital (A/D) converter, and the A/Dconverter provides the digital signal to a look-up table that determinesa display value (or values) representing the pressure (or force). A usermay use the display value as an absolute number and/or may move the kneeand compare pressure values at flexion and extension. The A/D converter,as well as any additional processing modules for processing sensed datainto usable data may all be housed in a processor (not shown). Theprocessor, in turn, may be housed in sensor plate 412 or in visualdisplay 420. Alternative methods for sensing and displaying sensed dataare also contemplated.

Sensor plate 412 is coupled with visual display 420 via cord 418, oralternatively via one or more other connection devices. In alternativeembodiments, for example, sensor plate 412 may be coupled with visualdisplay 420 via wireless infrared, electromagnetic, optical or otherremote, wireless connection(s). In various embodiments, sensors 430themselves may be coupled with visual display 420, or alternatively,sensors may be coupled with a processor housed in sensor plate 412, andthe processor (not shown) may then be coupled with visual display 420via cord 418 or other means. Visual display 420 itself may be attacheddirectly to sensor plate 412 or may be separate from sensor plate 412,as shown. In various embodiments, visual display 420 may be coupled withthe lower leg L or the thigh (not shown) of a patient via a strap 422 orother coupling means. As stated previously, visual display 420 may housea processor for processing sensed data transmitted from sensors 430 intousable data for displaying on LED readouts 424 or other display means.

FIG. 7 is an angled, perspective view of tibial portion 402 and othertibial components of system 400, as in FIGS. 5 and 6.

An alternative embodiment is shown in FIG. 8. As illustrated, tibialportion 402 may in some embodiments be attached to an immediatelyadjacent visual display 440 having multiple LED readouts. Either thesensors, the processor (neither are visible) or both are coupled withvisual display 440. In some embodiments, visual display 440 is coupledwith both sensor plate 412 and adaptor 414. Generally, visual display440 may have any suitable size, shape and overall configuration and maybe positioned in any appropriate location, relative to the rest ofsystem 400.

It is contemplated that any of the devices, systems and methodsdescribed above may be incorporated with any suitable knee surgeryprocedures or systems currently used or discovered in the future. Forexample, inventive devices, systems and methods may be readilyincorporated with any number of different visualization, navigationand/or robotic systems for performing a knee surgery, such asimage-guided systems for performing, planning or enhancing a TKAprocedure, robotic surgery systems such as the da Vinci® Surgical Systemprovided by Intuitive Surgical, Inc. (Sunnyvale, Calif.), or the like.Any suitable imaging or visualization modality and technique may be usedwith various embodiments of the devices, systems and methods of theinvention, such as but not limited to infrared or ultrasound imaging.Many suitable modifications and additions to the devices described abovemay also be made without departing from the scope of the invention.

Therefore, while the foregoing is a complete and accurate description ofexemplary embodiments of the present invention, various embodiments ofthe devices, systems and methods described may include any number ofmodifications and additions. The exemplary descriptions above shouldthus not be interpreted to limit the scope of the invention as it isdefined in the appended claims.

1. A method for sensing pressure in a knee during a surgical procedureon the knee, the method comprising: engaging femoral and tibial portionsof a knee adjustment device with the knee; sensing pressure exerted bythe femoral and tibial portions against one another using at least onesensor coupled with at least one of the femoral or tibial portion;displaying data describing the sensed force; and adjusting at least oneof the rotational orientation or anterior/posterior orientation of thefemoral portion of the knee adjustment device to separately adjustdynamic tension in the lateral and medial collateral ligaments adjacentthe knee, wherein adjusting affects the force exerted by the femoral andtibial portions against one another, wherein a stationary femoral memberof the femoral portion of the knee adjustment device engages a distalfemur of the knee and a mobile femoral member of the knee adjustmentdevice is moveably coupled to the stationary femoral member, and whereinadjusting at least one of the rotational orientation oranterior/posterior orientation of the femoral portion of the kneeadjustment device comprises moving the mobile femoral member relative tothe stationary femoral member.
 2. A method as in claim 1, furthercomprising moving the knee through a range of motion from a flexedposition to an extended position while the knee adjustment deviceremains engaged with the knee.
 3. A method as in claim 2, wherein theadjusting step is performed before the moving step.
 4. A method as inclaim 2, wherein the adjusting step is performed after the moving step.5. A method as in claim 2, wherein the adjusting step is performedbefore and after the moving step.
 6. A method as in claim 2, wherein theadjusting step is performed one or more times to balance force datadisplayed while the knee is flexed with force data displayed while theknee is extended.
 7. A method as in claim 2, further comprisingproviding positional data for helping position at least part of aprosthetic knee, wherein the adjusted femoral portion provides thepositional data.
 8. A method as in claim 1, further comprisingprocessing the sensed force into the data describing the force.
 9. Amethod as in claim 8, wherein processing comprises converting analogdata to digital data.
 10. A method as in claim 1, wherein sensing theforce comprises: transmitting a voltage to the at least one sensor;measuring the voltage after it has passed through the sensor(s);determining a percentage of the voltage passed through the sensor(s)relative to the voltage transmitted to the sensors; and deriving thepressure from the percentage.
 11. A method as in claim 1, whereinsensing the force comprises sensing with at least one sensor selectedfrom the group consisting of piezoelectric sensors, force sensingresistors, strain gauges, load cells, other pressure sensors and otherforce sensors.
 12. A method as in claim 1, wherein displaying the datacomprises: displaying at least a first number representing the pressurein a medial portion of the adjustment device; and displaying at least asecond number representing pressure in a lateral portion of theadjustment device, wherein a user separately adjusts the medial andcollateral tensions based on the displayed medial and collateral force.13. A method as in claim 12, wherein the first and second numbersrepresent relative amounts of force as compared to one another or toforce(s) measured with different positioning of the knee.
 14. A methodas in claim 12, wherein the first and second numbers represent absoluteamounts of force.
 15. A method as in claim 1, further comprising:receiving an input from a user of a desired amount of force to beexerted by the femoral and tibial members against one another; andautomatically adjusting the femoral portion of the adjustment device toachieve the desired amount of force.
 16. A method as in claim 15,wherein receiving the input comprises receiving a first amount ofdesired force for a medial side of the knee and a second amount ofdesired force for a lateral side of the knee.
 17. A method as in claim1, further comprising automatically adjusting the adjustment device tobalance the force exerted by the femoral and tibial members against oneanother while the knee is flexed with force exerted while the knee isextended.
 18. A method as in claim 1, further comprising cutting thedistal end of the femur to produce a cut surface, wherein the kneeadjustment device is engaged against said cut surface.
 19. A method asin claim 1, further comprising moving the knee through a range of motionfrom an extended position to a flexed position while the knee adjustmentdevice remains engaged with the knee.
 20. A method as in claim 2 or 19,wherein the patella of the knee remains in its anatomical position whilethe knee is moved through the range of motion.
 21. A method as in claim1, wherein moving the mobile femoral member relative to the stationaryfemoral member comprises separately moving laterally opposite sides ofthe mobile femoral member relative to the stationary femoral member. 22.A method as in claim 21, wherein the laterally opposite sides of themobile femoral member comprises a medial side contacting the medialposterior condyle of the distal femur and a lateral side contacting thelateral posterior condyle of the distal femur.
 23. A method as in claim2, wherein a first surface of the femoral portion contacts the tibialportion when the knee is in the extended position, and wherein aposterior condylar surface of the femoral portion contacts the tibialportion when the knee is in the flexed position, the posterior condylarsurface being disposed at a right angle relative to the first surface.